DeMartini et al.: Validated morphological metric for lobster size at maturity 



25 



Female, Scyllandes squammosus, 56.0 mm TW 

 29.8 mm exopodite length 



5mm 



exopodite 



B 



5mm 



Female, Panulirus marginalus, 51 .2 mm TAW 

 36.1 mm exopodite length 



endopodite 



egg-bearing 

 setae 



exopodite 



endopodite 



egg-bearing 

 setae 



Figure 1 



Schematic diagram of the left first exopodite (ventral aspect! of (A) slipper lobster (Scyllarides squammosus) and (B) the 

 Hawaiian spiny lobster I Panulirus marginatus), showing axis of measurement. TW = tail width. 



tological scores in assessing gonadal maturity 

 (Minagawa and Sano, 1997). Gonadal maturity 

 was used as a means of validating, as well as ref- 

 erencing, estimates of size at functional maturity 

 (Ennis, 1984). 



Statistical analyses 



Data for EL and EW (as response variables) and 

 TW (regressor) for the same specimen were first 

 plotted for all specimens of each species. Prelimi- 

 nary evaluations of these data (both raw and log- 

 transformed) with least-squares linear regression 

 (REG procedure; SAS vers. 8, SAS Institute, Inc., 

 Cary, NC) indicated allometric relationships for 

 which double-log functions provided approximate 

 fits. Identification of join points by iteration based 

 on minimizing the total residual sums of squares 

 of pairs of joined regression equations (Somerton, 

 1980), however, resulted in linear spline fits that, 

 although significant, had obviously nonrandom 

 residuals. Simple linear fits with log-log plots, how- 

 ever, were useful for selecting the most appropriate 

 metric: the regressions of EW on TW, qualitatively 

 similar to those for EL regressed on TW, had con- 

 sistently lower r 2 values, likely because pleopod 

 width was more difficult to measure than pleopod 

 length. The EL metric was therefore chosen for all 

 further analyses. 



Because lobsters, like most biological populations, are 

 composed of individuals that differ in the size at which 

 first maturity occurs, we fitted a curve to the EL-TW re- 

 lation that included a sigmoid segment bridging the re- 

 gion between the estimated sizes of the smallest adults 

 (0 O ) and the largest immature individuals (0j) (Fig. 2). 

 The curve was fitted by using iterative reweighted least 



squares (S-Plus 6 for Windows, Insightful Corporation, 

 Seattle, WA; Ratkowsky, 1983) with appropriate weights 

 to standardize the variance (Appendix). The morphomet- 

 ric maturation point, hereafter referred to as the MMP, 

 was estimated at the inflection ([0Q+0J/2) of the sigmoid 

 segment of the curve. This inflexion point represents 

 the body size at which we expect 50% of the lobsters to 

 become sexually mature (median size at attainment of 



